WO2011080838A1 - Absorbing solution and recovery method for carbon dioxide - Google Patents

Abstract

A method for the recovery of carbon dioxide (CO₂) which comprises: bringing, in an absorption tower, a gas to be treated into contact with a CO₂-absorbing solution set forth in claim 1 to form a CO₂-rich solution, said gas containing both CO₂ and oxygen; circulating the CO₂-rich solution in a regeneration tower to heat and discharge the CO₂ and thus recover the same; returning the obtained CO₂-poor solution to the absorption tower; and conducting heat exchange between the solution transferred from the absorption tower to the regeneration tower and the solution returned from the regeneration tower to the absorption tower. The method is characterized by: adding either a silicone oil and/or an aqueous alkanolamine solution in which an organosulfur compound represented by general formula (A) or (B) is dissolved to the solution present in the absorption tower and/or the solution returned from the regeneration tower to the absorption tower; and controlling the composition of the absorbing solution present in the absorption tower in a manner that makes the content of the alkanolamine falls within the range of 30 to 60wt%, that makes the content of the organosulfur compound falls within the range of 0.01 to 2wt%, and that makes the content of the silicone oil falls within the range of 5 to 100ppm (by weight).

Description

Carbon dioxide absorption liquid and recovery method

TECHNICAL FIELD The present invention relates to a carbon dioxide absorption liquid and a carbon dioxide recovery method using the same, and in particular, a carbon dioxide absorption liquid and recovery for recovering carbon dioxide stably from exhaust gas from a combustion apparatus such as a boiler for a long period of time. It concerns the method.

In thermal power plants and the like, carbon dioxide (hereinafter referred to as CO ₂ ) is generated with the combustion of fossil fuels such as coal, increasing the concentration of CO ₂ in the atmosphere. It has been said that problems will arise. In order to prevent global warming, since the Kyoto Protocol was adopted by the Kyoto Conference on Global Warming Prevention (COP3) in December 1997, measures to reduce CO ₂ emissions have been implemented in various countries. Among the methods for recovering CO ₂ from combustion exhaust gas containing oxygen (O ₂ ) and sulfur oxides (SO _X ) in thermal power plants, the absorption method using an alkanolamine solution is the most practical method at present. (Patent Document 1). In this regard, studies on types of alkanolamines suitable for CO ₂ recovery (for example, Patent Document 2) and conditions for application to exhaust gas containing sulfur oxides (for example, Patent Document 1) have been conducted. These alkanolamines are not easily oxidized by oxygen contained in combustion exhaust gas as compared with the conventionally used monoethanolamine, but considering the long-term use over several thousand hours, The amount of replenishment due to oxidative degradation has become a problem. On the other hand, by adding an organic sulfur compound such as mercaptoimidazoles or mercaptobenzimidazoles as an alkanolamine oxidation inhibitor (for example, Patent Document 3), the oxidation of the alkanolamine in the absorbing solution is greatly increased. It became possible to suppress.

As a method for recovering CO ₂ contained in the combustion exhaust gas of the boiler, a gas to be treated containing CO ₂ is brought into contact with an alkanolamine absorbing solution in an absorption tower to obtain a solution rich in CO _2. A method of circulating CO ₂ and recovering CO ₂ by heating and releasing CO ₂ is performed (for example, Patent Document 1).

Patent No. 3529855 specification Patent No. 2871334 specification Japanese Patent No. 3739437

In the carbon dioxide recovery method described above, the alkanolamine in the absorbent is partially oxidized / decomposed by oxidizing components such as O ₂ and NOx in the combustion exhaust gas, particularly oxygen. Addition can suppress the oxidative decomposition of alkanolamine, but it has been found that the effect of the oxidation inhibitor is not sufficiently exhibited depending on the operating conditions of the absorption tower.

That is, the present inventors have found that the oxidation inhibitor in the absorption liquid is oxidized by the following phenomenon in the absorption tower, so that the concentration of the oxidation inhibitor is lowered and the alkanolamine is easily oxidized. That is, in the absorption tower, in order to increase the amount of CO ₂ recovered, the exhaust gas and the absorption liquid are contacted with a packed bed using a high specific surface area filler so that the contact area between the absorption liquid and the combustion exhaust gas can be made as large as possible. Then, bubbles are easily formed when the absorption liquid or combustion exhaust gas passes through a relatively narrow flow path, and when the bubbles are broken, particles whose settling velocity is lower than the gas flow velocity are scattered along with the gas. . The scattered particulate absorption liquid (absorption liquid mist) has a larger specific area in contact with air than the bulk absorption liquid, so that the oxidation inhibitor in the absorption liquid is oxidized, and the concentration in the liquid decreases. As a result, it was found that the alkanolamine in the absorbent mist is easily oxidized.

In order to prevent this, it is conceivable to increase the concentration of the oxidation inhibitor. However, when the oxidation inhibitor is difficult to dissolve in water, such as an organic sulfur compound, the addition amount is limited and the oxidation inhibitor There is a problem that the amount of consumption increases.

An object of the present invention is to suppress the oxidation of the oxidation inhibitor in the absorption liquid without increasing the concentration of the oxidation inhibitor in the CO ₂ absorption liquid containing alkanolamine, thereby An object of the present invention is to provide a CO ₂ absorption method that can reduce the amount of scattering from the absorption tower due to oxidative decomposition and reduce the replenishment amount.

In order to solve the above problems, the invention claimed in the present application is as follows.
(1) An absorption liquid capable of absorbing and releasing carbon dioxide from a gas to be treated containing carbon dioxide and oxygen, the absorption liquid being C _n H _{2n + 1} NHC _{n ′} H _{2n ′ + 1} O (N is an arbitrary integer of 0 to 4, n ′ is an arbitrary integer of 1 to 3), a mercaptoimidazole represented by the following formula (A), and a formula (B) One or two or more organic sulfur compounds selected from the group consisting of mercaptobenzimidazoles, and an aqueous solution containing silicone oil, wherein the aqueous solution contains 30% by weight or more and 60% by weight or less of the alkanolamine. A carbon dioxide absorbing solution comprising an organic sulfur compound in an amount of 0.01 weight percent to 2 weight percent and the silicone oil in an amount of 5 ppm to 100 ppm (weight ratio).

Here, R ₁ , R ₂ and R ₃ are a hydrogen atom, a C ₁ -C ₃ alkyl group, a phenyl group or a benzyl group, R ₄ is a hydrogen atom or a C ₁ -C ₃ alkyl group, n is 1 It is an integer of ~ 3.
(2) The alkanolamine is selected from 2-aminoethanol, 2- (methylamino) ethanol, 2- (ethylamino) ethanol, 2-amino-2-methyl-1-propanol, and 2- (isopropylamino) ethanol. The carbon dioxide absorption liquid according to (1), which is one or more compounds selected from the group consisting of:
(3) A gas to be treated containing carbon dioxide (CO ₂ ) and oxygen is brought into contact with the CO ₂ absorbing solution according to claim 1 in an absorption tower to obtain a solution rich in CO _2. with a solution circulates to recover by heating releases CO _2, recycled to the absorption tower as a solution poor in CO _2, and a liquid for transporting to the regenerator from the foregoing absorption column, the absorption tower from the regeneration tower A method for recovering carbon dioxide by exchanging heat with a liquid to be recycled, wherein the liquid inside the absorption tower and / or the liquid recycled from the regeneration tower to the absorption tower is silicone oil and / or the above formula (A) or (B) An alkanolamine aqueous solution in which the organic sulfur compound is dissolved is added, and the composition of the absorbing solution in the absorption tower is 30 wt% or more and 60 wt% or less, and the organic sulfur compound is 0.01 wt% or less. , 2 wt% or less, and the silicone oil 5ppm above method for recovering carbon dioxide and adjusts so that 100 ppm (or less by weight).
(4) The alkanolamine is selected from 2-aminoethanol, 2- (methylamino) ethanol, 2- (ethylamino) ethanol, 2-amino-2-methyl-1-propanol, and 2- (isopropylamino) ethanol. The method for recovering carbon dioxide according to (3), which is one or more compounds selected from the group consisting of:
(5) The alkanolamine aqueous solution in which the organic sulfur compound of the formula (A) or (B) is dissolved has an alkanolamine concentration equal to or higher than the alkanolamine concentration in the absorbent, and the concentration of the organic sulfur compound is also in the absorbent. (2) The method for recovering carbon dioxide according to (3) or (4) above, wherein the organic sulfur compound concentration is not less than

According to the CO ₂ absorbing liquid and the CO ₂ recovery method of the present invention, it is possible to suppress the oxidative decomposition of the alkanolamine and the oxidation inhibitor contained in the absorbing liquid, and as a result, the alkanol in the absorbing liquid during operation. The amount of scattering due to oxidative decomposition of amines and oxidation inhibitors can be reduced, and the replenishment amount can be reduced.

The systematic diagram of the experimental apparatus used in the Example of this invention.

1: humidifier, 2: cooler, 3: heat exchanger, 4: preheater, 5: amine cooler, 6: gas monitor, 7: absorption tower, 8: regeneration tower

The CO ₂ absorbing solution of the present invention is represented by C _n H _{2n + 1} NHC _{n ′} H _{2n ′ + 1} O (n is an arbitrary integer from 0 to 4, and n ′ is an arbitrary integer from 1 to 3). Alkanolamines, particularly 2-aminoethanol (hereinafter sometimes referred to as MEA, molecular weight: 61.08), 2-methylaminoethanol (hereinafter sometimes referred to as MAE, molecular weight: 75.12), 2- Ethylaminoethanol (hereinafter sometimes referred to as EMEA, molecular weight: 89.14), 2-amino-2-methyl-1-propanol (hereinafter sometimes referred to as AMP, molecular weight: 89.14), 2-isopropylamino One or a mixture of two or more of ethanol (hereinafter sometimes referred to as IPAE, molecular weight: 103.16) is preferable from the viewpoint of the high CO ₂ absorption rate and the low heat of dissociation of absorbed CO ₂ .

The concentration of alkanolamine in the CO ₂ absorbent is 35 to 60% by weight, and the optimum concentration varies depending on the apparatus. When using a general absorption tower using 30% by weight MEA aqueous solution (for example, the example of Japanese Patent Publication No. 5-87290) under the same usage conditions as MEA aqueous solution, the molar concentration of amine is taken into consideration. For example, if EMEA and AMP are adjusted to 44% by weight, IPAE to 51% by weight, EMEA and IPAE are equimolar, EMEA is 22% by weight, and IPAE is 25% by weight. Good. The lower the alkanolamine concentration is, the lower the viscosity becomes, and there is an operational merit that it becomes difficult to flood the packed bed in the absorption tower. However, the lower the concentration, the lower the amount of CO ₂ absorbed per unit liquid, There is a demerit that the CO ₂ absorption performance in the absorption tower and the heat supply amount at the time of CO ₂ recovery in the regeneration tower increase.

As the silicone oil in the CO ₂ absorbent of the present invention, there are various types such as a hydrophilic oil type, a solvent type obtained by adding a solvent to silicone, an emulsion type, a powder type, and a self-emulsifying type. Oil type and self-emulsifying type are preferred. Furthermore, it is desirable to have a certain level of heat resistance in consideration of heating in the regeneration tower while maintaining an emulsion state even when added to an alkaline aqueous solution. As silicone oils satisfying such properties, KS604, KS537 and KS538 of Shin-Etsu Silicone, FS544 of Toray Dow Corning, etc. can be used. Silicone oil that forms granular precipitates is not desirable because it may promote the foaming of the amine liquid.

The concentration of the silicone oil is in the range of 5 to 100 ppm, preferably 10 to 50 ppm. When the concentration of silicone oil is less than 5 ppm, the generation of absorbent mist may not be sufficiently suppressed. On the other hand, when the concentration is higher than 100 ppm, the generation of the absorption liquid mist can be sufficiently suppressed, but the CO ₂ absorption reaction may be inhibited by covering the surface layer of the absorption liquid with silicone oil.

Examples of the mercaptoimidazoles of the organic acid sulfur compound of the formula (A) include 2-mercaptoimidazole, 2-mercapto-1-methylimidazole, 2-mercapto-4-methylimidazole, 2-mercapto-5-methylimidazole, 2 -Mercapto-1,4-dimethylimidazole, 2-mercapto-1,5-dimethylimidazole, 2-mercapto-1-phenylimidazole, 2-mercapto-4-phenylimidazole, 2-mercapto-1-benzylimidazole, etc. It is done. Examples of the mercaptobenzimidazoles of the above formula (B) include 2-mercaptobenzimidazole, 4-methyl-2-mercaptobenzimidazole, 5-methyl-2-mercaptobenzimidazole and the like.

The concentration of the organic sulfur compound in the CO ₂ absorbing solution is 0.01 to 2% by weight, preferably 0.1 to 1% by weight. However, if it is less than 0.01% by weight, the oxidation inhibiting effect becomes low and is not practical. When it exceeds, since it will become a density | concentration close | similar to saturation solubility, an organic sulfur compound may precipitate by the influence of the trace component in waste gas.

In addition, the present inventors have found that the organic sulfur compound is soluble in alkanolamine. Accordingly, an aqueous solution having a higher alkanolamine concentration than the absorbing solution can dissolve an organic sulfur compound having a higher concentration than that of the absorbing solution. It becomes possible to adjust the concentration of the sulfur compound therein.

In the region where the combustion exhaust gas in the absorption tower is in contact with the absorption liquid, in order to suppress the generation of the absorption liquid mist, the organic sulfur compound and the silicone oil of the predetermined concentration need to be included. In particular, when the absorption liquid is heated in the regeneration tower, the silicone oil is more likely to be altered and the effect is likely to be lower than that of the organic sulfur compound, so the concentration of the silicone oil in the absorption liquid in the absorption tower is within a predetermined range. It is important to keep it within. Therefore, an alkanolamine aqueous solution in which silicone oil and / or an organic sulfur compound is dissolved is added to the absorption liquid recirculated from the regeneration tower to the absorption tower or the absorption liquid in the absorption tower, and these have a predetermined concentration in the absorption tower. It is preferable to adjust so that it may be maintained. In the case of starting the CO ₂ recovery apparatus, an aqueous alkanolamine solution alone, or after the aqueous alkanolamine solution containing a silicone oil or organic sulfur compounds was filled in an absorption tower, adding an absorbent solution containing an additive of a given concentration of present invention You may do it.

FIG. 1 is an explanatory diagram of a CO ₂ absorption device showing one embodiment of the present invention. The apparatus includes a CO ₂ absorption tower 7 in which the processing gas and the CO ₂ absorbing liquid to gas-liquid contact, the CO ₂ by heating the absorption liquid becomes CO ₂ rich absorbs CO ₂ in the absorption tower 7 It mainly comprises a regeneration tower 8 that separates and recovers and regenerates the absorbing liquid. The simulated combustion exhaust gas containing O ₂ and N _{2 in} addition to CO ₂ is supplied to the absorption tower 7 through the humidifier 1 and comes into contact with the absorption liquid flowing down from the top of the absorption tower 7 while the gas rises. CO ₂ is absorbed. The exhaust gas is collected in the water cooling section installed in the upper part of the tower, and then guided to the gas outlet line. After passing through the cooler 2, the exhaust gas is discharged to the atmosphere. Part of the exhaust gas is sent to the gas monitor 6 for analysis. Absorbing liquid rich in CO ₂ (rich absorbing liquid) exiting from the absorption tower 7 exits the absorption tower 7 from the liquid outlet line 9 at the bottom of the tower, passes through the pump 10, is guided to the heat exchanger 13, and The temperature is raised through the preheater 4. Thereafter, the rich absorbent is supplied to the regeneration tower 8 whose outer wall 11 is heated by a heater, and releases CO ₂ here. The released CO ₂ passes through the top outlet line of the regeneration tower 8, passes through the condenser 12, and is released to the atmosphere. Here, the condensate (lean absorbent) returns to the regeneration tower 8, flows down the regeneration tower, passes through the bottom of the regeneration tower, reaches the heat exchanger 13 by the pump 14, and exchanges heat with the rich absorbent here. Then, after passing through the amine cooler 5, it is recycled to the absorption tower 7.

The tower diameter of the absorption tower 7 and the regeneration tower 8 used in this example were both 50 mm, and the packed bed height of the packing was 1.4 m. As the silicone oil, commercially available names such as KS604, KS537, and KS538 of Shin-Etsu Silicone and FS544 of Toray Dow Corning were used, but the present invention is not limited to these.

The main experimental conditions are absorption tower inlet gas and liquid temperature 30 ° C., regeneration tower inlet liquid temperature 100 ° C., regeneration tower liquid temperature: maximum 110 ° C., cooler temperature 30 ° C., gas amount 2 m ³ / h, liquid gas ratio 3. 0 (L / m ³ ).

In this example, 2-ethylaminoethanol (EMEA), 2-aminoethanol (MEA), and 2-isopropylaminoethanol (IPAE) were used as alkanolamines, and ammonia, ethylamine, and isopropylamine were used as the respective alkanolamines. It was detected as a decomposition product and evaluated for oxidation stability.

Specifically, the gas was branched from the outlet gas of the absorption tower 7 by 1 L / min and passed through an absorption bottle containing 100 ml of 0.01 N HCl for 1 h, and then the concentration of decomposition products in the liquid was measured by ion chromatography. Measured from this, the amount of alkanolamine oxidation / decomposition per CO ₂ recovery amount was determined, and as described later, the amount of scattering in each example was set to 1, and the relative value of the amount of scattering in the corresponding comparative example as 1. It showed in.

Incidentally, the amount of scattered as a reference of CO ₂ gas recovery amount was calculated by the following method.
(1) Calculate the CO ₂ gas recovery amount (m ³ / h) from the assumption that the measured values C1 and C2 of the CO ₂ concentration in the inlet gas and the outlet gas and the change in the inlet / outlet gas amount = CO ₂ recovery amount .

This value is then multiplied by 1000 (L / m ³ ) /22.4 (L / mol) x 44 (g / mol) / 1000 (kg / g) to calculate the CO ₂ gas recovery amount (kg / h). To do.
(2) Sort until the outlet gas volume reaches 60 L, and measure the suction gas volume with a dry gas meter (DC-1 type, manufactured by Shinagawa).
(3) The concentration of ammonia and the specific decomposition amine (no MEA, methylamine for MAE, ethylamine for EMEA, isopropylamine for IPAE) in the absorption bottle through which the suction gas has passed Measured with a graph (DX-100 manufactured by DIONEX), the total number of moles is taken as the decomposition product concentration, and the total decomposition product amount (per 60 L of gas) is calculated from 100 ml of the absorption liquid.
(4) The amount of decomposition product of (3) is converted to the total exhaust gas amount (2m ³ -CO ₂ recovery amount) from the absorption tower, and the amount is the amount of scattering in each example and comparative example.
(5) The amount of scattering that is the CO ₂ recovery amount standard is calculated by dividing the amount of scattering per exhaust gas by the amount of CO ₂ recovery (kg / h) of each example and comparative example.

Example 1
Using the apparatus of FIG. 1, the concentration of 2-ethylaminoethanol is 45 weight percent, the concentration of 2-mercaptobenzimidazole as the organic sulfur compound is 0.1 weight percent, and the concentration of KS604 from Shin-Etsu Silicone as the silicone oil is 5 ppm ( mg / kg) of CO ₂ absorbing solution and CO ₂ absorption and recovery experiment from exhaust gas under the above-mentioned conditions.
Example 2
The experiment was performed under the same conditions as in Example 1 except that the concentration of 2-mercaptobenzimidazole was 0.01 weight percent.
Example 3
The experiment was performed under the same conditions as in Example 1 except that the concentration of 2-mercaptobenzimidazole was 2.0 weight percent.

Example 4
The experiment was performed under the same conditions as in Example 1 except that the concentration of the silicone oil was 50 ppm (mg / kg).
Example 5
The experiment was performed under the same conditions as in Example 1 except that the concentration of the silicone oil was 100 ppm (mg / kg).
Example 6
The experiment was performed under the same conditions as in Example 1 except that the concentration of 2-ethylaminoethanol was 30 weight percent.

Example 7
The experiment was performed under the same conditions as in Example 1 except that the concentration of 2-ethylaminoethanol was 60 weight percent.
Example 8
The experiment was performed under the same conditions as in Example 1 except that 0.1% by weight of 2-mercapto-1-methylimidazole was added as an organic sulfur compound.
Example 9
The experiment was performed under the same conditions as in Example 1 except that 0.01 weight percent of 2-mercapto-1-methylimidazole was added as the organic sulfur compound.

Example 10
The experiment was conducted under the same conditions as in Example 1 except that 2.0 weight percent of 2-mercapto-1-methylimidazole was added as an organic sulfur compound.
Example 11
The experiment was performed under the same conditions as in Example 1 except that 0.1 weight percent of 2-mercapto-5-methylbenzimidazole was added as an organic sulfur compound.
Example 12
The experiment was conducted under the same conditions as in Example 1 except that 0.01% by weight of 2-mercapto-5-methylbenzimidazole was added as the organic sulfur compound.

Example 13
The experiment was performed under the same conditions as in Example 1 except that 2.0 weight percent of 2-mercapto-5-methylbenzimidazole was added as an organic sulfur compound.
Example 14
The experiment was performed under the same conditions as in Example 1 except that 30% by weight of 2-aminoethanol was added as an alkanolamine.
Example 15
The experiment was performed under the same conditions as in Example 2 except that 30% by weight of 2-aminoethanol was added as an alkanolamine.

Example 16
The experiment was performed under the same conditions as in Example 3 except that 30% by weight of 2-aminoethanol was added as an alkanolamine.
Example 17
The experiment was performed under the same conditions as in Example 1 except that 52% by weight of 2-isopropylaminoethanol was added as an alkanolamine.
Example 18
The experiment was performed under the same conditions as in Example 2 except that 52% by weight of 2-isopropylaminoethanol was added as an alkanolamine.

Example 19
The experiment was performed under the same conditions as in Example 3 except that 52% by weight of 2-isopropylaminoethanol was added as an alkanolamine.
Example 20
The experiment was conducted under the same conditions as in Example 1 except that 2-ethylaminoethanol and 2-isopropylaminoethanol were added as alkanolamines at 22.5 and 26 weight percent, respectively.
Example 21
The experiment was conducted under the same conditions as in Example 2 except that 2-ethylaminoethanol and 2-isopropylaminoethanol were added as alkanolamines at 22.5 and 26 weight percent, respectively.

Example 22
The experiment was performed under the same conditions as in Example 3 except that 22.5 and 26 weight percent of 2-ethylaminoethanol and 2-isopropylaminoethanol were added as alkanolamines, respectively.
Example 23
The experiment was performed under the same conditions as in Example 1 except that 2-ethylaminoethanol and 2-aminoethanol were added as alkanolamines at 22.5 and 15 weight percent, respectively.
Example 24
The experiment was carried out under the same conditions as in Example 2 except that 2-ethylaminoethanol and 2-aminoethanol were added as alkanolamines at 22.5 and 15 weight percent, respectively.

Example 25
The experiment was performed under the same conditions as in Example 3 except that 22.5 and 15 weight percent of 2-ethylaminoethanol and 2-aminoethanol were added as alkanolamines, respectively.
Example 26
The experiment was conducted under the same conditions as in Example 1 except that 15-26 weight percent of 2-aminoethanol and 2-isopropylaminoethanol were added as alkanolamines, respectively.
Example 27
The experiment was performed under the same conditions as in Example 2 except that 15-26 weight percent of 2-aminoethanol and 2-isopropylaminoethanol were added as alkanolamines, respectively.

Example 28
The experiment was performed under the same conditions as in Example 3, except that 15 and 26 weight percent of 2-aminoethanol and 2-isopropylaminoethanol were added as alkanolamines, respectively.
Example 29
The experiment was performed under the same conditions as in Example 1 except that KS537 manufactured by Shin-Etsu Silicone Co., Ltd. was used as the silicone oil.
Example 30
The experiment was performed under the same conditions as in Example 1 except that KS538 manufactured by Shin-Etsu Silicone Co., Ltd. was used as the silicone oil.
Example 31
The experiment was performed under the same conditions as in Example 1 except that Toray Dow Corning FS544 was used as the silicone oil.

Comparative Example 1
The experiment was performed under the same conditions as in Example 1 except that no silicone oil was added.
Comparative Example 2
The experiment was performed under the same conditions as in Example 1 except that no organic sulfur compound and silicone oil were added.
Comparative Example 3
The experiment was performed under the same conditions as in Example 1 except that 2-mercaptobenzimidazole was not added.
Comparative Example 4
The experiment was performed under the same conditions as in Example 1 except that the concentration of the silicone oil was 200 ppm (mg / kg).

Comparative Example 5
Silicone oil was not added. Except this, the conditions were the same as in Example 2.
Comparative Example 6
The experiment was performed under the same conditions as in Example 3 except that no silicone oil was added.
Comparative Examples 7 to 29
Experiments were performed under the same conditions as in Examples 6 to 28 except that no silicone oil was added.
Comparative Example 30
The experiment was conducted under the same conditions as in Example 1 except that polypropylene glycol (trade name PPG-700, manufactured by Nippon Oil Chemical Co., Ltd.), an organic surfactant, was added to the absorbent so as to be 100 ppm instead of silicone oil. It was.

Example 32
Simulating the case where silicone oil deteriorates and has no effect, while operating under the same conditions as in Comparative Example 1, the absorption liquid after passing through the amine cooler 5 of the apparatus of FIG. The diluted aqueous solution was supplied, and at that time, the flow rate was adjusted with a micropump so that the concentration of the silicone oil in the absorbent circulated to the absorption tower 7 was 5 ppm.

Example 33
Simulating the case where the organic sulfur compound is oxidized and has no effect, and operating under the same conditions as in Comparative Example 3, 1 weight percent 2-mercaptobenzimidazole, 45 weight percent 2-ethylaminoethanol and 5 ppm Shin-Etsu Silicone The absorption liquid containing KS604 was supplied to the absorption liquid after passing through the amine cooler 5 of the apparatus of FIG. 1 at a flow rate of 10% of the absorption liquid. At that time, the circulation amount of the absorbing liquid was adjusted so that the liquid gas ratio was 3.0.

The conditions and evaluation results of the above examples and comparative examples are shown in Tables 1 to 3, respectively. The evaluation results are as follows. From the analysis of the decomposition products in the exhaust gas from the absorption tower and the recovery gas from the regeneration tower, the amount of oxidation / decomposition product of alkanolamine per CO ₂ recovery amount was determined. The amount of scattering in the corresponding comparative example is taken as 1, and the relative value is shown. In Table 3, the symbol ◎ indicates that the amount of scattering decreased to less than 0.5 times the amount of oxidative decomposition product of the corresponding comparative example, and the symbol ○ indicates 0.5 times the amount of scattering of the corresponding comparative example. As mentioned above, it shows that the amount of scattering has decreased to less than 0.7 times, and the Δ mark indicates that the amount of scattering has decreased to 0.7 times or more and less than 0.9 times the amount of scattering in the corresponding comparative example. Indicates.

From the results of Table 3, in the comparison between Example 1 and Comparative Example 2, the oxidation of alkanolamine was clearly suppressed due to the inclusion of the organic sulfur compound, but the alkanolamine was clearly suppressed by the addition of silicone oil. It can be seen that the oxidation of is suppressed. Moreover, it turns out that there is no effect even if it adds a silicone oil excessively from the result of the comparative example 4. When the CO ₂ recovery rate was compared in Comparative Example 4 and Example 1 at the same liquid gas ratio 3, Comparative Example 4 always showed a lower value than Example 1. As described above, it can be presumed that the CO ₂ absorption reaction was inhibited by covering the surface layer of the absorbing solution with the silicone oil. Moreover, although it was an organic type surfactant of the comparative example 30, the added effect was not seen by the same concentration as silicone oil.

Next, when the result of Example 32 was compared with Example 1, the amount of oxide was equal, and it was shown that the same effect can be obtained by the method of claim 3.

Also, the oxide amount in the method of Example 33 is equal to that in Example 1, and it was shown that the same effect can be obtained by the method of claim 5.

Claims (5)

An absorption liquid capable of absorbing and releasing carbon dioxide from a gas to be treated containing carbon dioxide and oxygen, wherein the absorption liquid is C _n H _{2n + 1} NHC _{n ′} H _{2n ′ + 1} O (n is Any integer of 0 to 4, n ′ is an integer of 1 to 3), mercaptoimidazoles represented by the following formula (A) and mercaptobenz represented by the formula (B) One or two or more organic sulfur compounds selected from the group consisting of imidazoles, and an aqueous solution containing silicone oil, wherein the aqueous solution contains 30% by weight or more and 60% by weight or less of the alkanolamine. Is contained in an amount of 0.01 weight percent or more and 2 weight percent or less, and the silicone oil is contained in an amount of 5 ppm or more and 100 ppm (weight ratio or less).

Here, R ₁ , R ₂ and R ₃ are a hydrogen atom, a C ₁ -C ₃ alkyl group, a phenyl group or a benzyl group, R ₄ is a hydrogen atom or a C ₁ -C ₃ alkyl group, n is 1 It is an integer of ~ 3. The alkanolamine is selected from the group consisting of 2-aminoethanol, 2- (methylamino) ethanol, 2- (ethylamino) ethanol, 2-amino-2-methyl-1-propanol, and 2- (isopropylamino) ethanol. 2. The carbon dioxide absorption liquid according to claim 1, wherein the carbon dioxide absorption liquid is one or two or more selected compounds. A gas to be treated containing carbon dioxide (CO ₂ ) and oxygen is brought into contact with the CO ₂ absorbing solution according to claim 1 in an absorption tower to obtain a CO ₂ -rich solution, which is subsequently circulated in the regeneration tower. CO ₂ is heated and released to be recovered and recycled to the absorption tower as a CO ₂ -poor solution, and recirculated from the absorption tower to the regeneration tower and from the regeneration tower to the absorption tower. A method for recovering carbon dioxide by heat exchange with a liquid, wherein the liquid inside the absorption tower and / or the liquid recycled from the regeneration tower to the absorption tower is silicone oil and / or the above formula (A) or (B) An alkanolamine aqueous solution in which the organic sulfur compound is dissolved is added, and the composition of the absorption liquid in the absorption tower is 30% by weight or more and 60% by weight or less, and the organic sulfur compound is 0.01% by weight or more and doubled. Percent, and the silicone oil 5ppm above method for recovering carbon dioxide and adjusts so that 100 ppm (or less by weight). The alkanolamine is selected from the group consisting of 2-aminoethanol, 2- (methylamino) ethanol, 2- (ethylamino) ethanol, 2-amino-2-methyl-1-propanol, and 2- (isopropylamino) ethanol. 4. The method for recovering carbon dioxide according to claim 3, wherein the compound is one or two or more selected compounds. As the alkanolamine aqueous solution in which the organic sulfur compound of the formula (A) or (B) is dissolved, the alkanolamine concentration is not less than the alkanolamine concentration in the absorbing solution, and the concentration of the organic sulfur compound is also the organic sulfur in the absorbing solution. The method for recovering carbon dioxide according to claim 3 or 4, wherein the concentration is higher than a compound concentration.

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